Walmart offers the Mophie 2,500mAh Juice Pack Portable Battery Pack in Black, model no. 2031 for $15.38. Opt for in-store pickup to dodge the $4.97 shipping fee. That's the lowest price we could find by $2. This external battery works with USB-based mobile devices and smartphones.
Don't mind a refurb? Breed via Amazon has it for $9.95 with free shipping for Prime members.